Most industrial devices for the measurement of flow rates in open channels are described in the "Water Measurement Manual" published by the U.S. Department of Interior and available from the U.S. Government Printing Office. Of the many available configurations the notched weir and the Parshall venturi flume are most widely used. The notched weir is basically a dam with an opening or notch in the upper edge through which the water flows. The water level height above the bottom of the notch is used to measure the flow rate. The notch width and the corresponding operating capacity can be adjusted with a sliding door. The water after flowing through the notch must "spring clear" of the flume to get accuracy better than 3%. The dam formed by the weir will catch debris and limits the use of notched weirs to relatively clean streams. In chemical water treatment of acid mine drainage, such as in coal preparation plants and in discharge streams from coal mining operations, there is a need for a self-cleaning flow measuring device which can pass debris, rocks and silt without problems. For such applications the Parshall venturi flume is often used. This is a converging channel followed by a constant area throat section which continues smoothly into a diverging channel section. The operating range is fixed because of the fixed throat width, which cannot be adjusted without creating a flow discontinuity and altering the discharge characteristics of the venturi.